Quantum Spin Dynamics of Molecular Spintronic Devices Based on Single-molecule Nanomagnets

© 2020 Kieran HymasIn recent years, molecular analogues to electronic devices have been sought after to remedy the practical limitations imposed on classical circuits by Moore's law leading to the inception of the multidisciplinary research field of molecular electronics. In addition to the miniaturisation of current electronic technologies, researchers have sought also to exploit the interplay between the spin degree of freedom inherent to magnetic molecules embedded in devices and to the local electronic currents to which they are coupled. Single-molecule magnets (SMMs), metal complexes with large magnetically anisotropic spin moments that exhibit slow relaxation effects, have enjoyed a position in the subfield of molecular spin-electronics (spintronics) as magnetic units that may act as elements in new molecular-scale spintronic technologies. In this thesis, three projects composed of theoretical models of spin transport through single-molecule magnet-based spintronic devices are presented which serve to predict and explain the quantum spin dynamics exhibited by novel device set-ups that are based on current state-of-the-art experimental systems. In the first project, I have contributed to the development of two models of spin-polarised transport through a general molecular nanomagnet device that is perturbed either by some time dependent, resonant perturbation or by a static perturbation. In the former case, a study of the time evolution of the quantum states of the nanomagnet revealed Rabi oscillations between spin states that are resonantly coupled by the perturbation, suggesting that these states could behave as a molecular qubit for quantum computation that is addressed with a spin-polarised current. In the steady-state limit of the time-dependent model the device functions as a spin current pump, amplifier and inverter which could be potentially useful for logic gates in novel circuitry based on the spin degree of freedom of an electronic current rather than on the charge; these effects are preserved in time-averaged current measurements of the device operating under a pulsed radiation regimen. In the second model, the spin inversion property of the device is preserved even when using a static rather than a time-dependent perturbation owing to a mixing between electric current blockaded and non-blockaded states. In the second project, I have contributed to the theoretical description of electron transport through a molecular break junction device housing a single terbium \begin{em}bis\end{em}-phthalocyaninato (TbPc$_2$) nanomagnet. The model developed in this project is shown to capture all experimental properties measured for the single-molecule device, in particular, its magneto-conductance dependence on the applied magnetic field, gate and bias voltage. Crucially, using the model it was possible to confirm that different states of the molecular magnet give rise to disparate signals in the magneto-conductance which may be used to perform an electrical read-out of the molecular states of the device. At variance with previous interpretations that advocated for a strongly coherent regime of electron transport through the device, the behaviour of the experimentally observed magneto-conductance is shown here to be fully captured within the incoherent sequential tunnelling regime. In the third project, I have contributed to the understanding of an experimentally realised molecular spin valve by providing the first simulations of the hysteresis of the differential magneto-conductance for a hybrid molecular-quantum point contact three-terminal device, triggered by the slow relaxation of the SMMs grafted to the device in a time-dependent sweeping magnetic field. The transport dynamics were modelled here in a completely incoherent transport regime without necessitating the tenuous assumption of spin dependent Fano-resonance interference that were invoked in previous theoretical studies of the devices. The signature of the slow relaxation of two or more TbPc$_2$ single-molecule magnets manifests in magneto-conductance measurements owing to a phonon-mediated direct relaxation process between the Tb electronic states leading to the transient population of non-conducting anti-parallel configurations of the TbPc$_2$ magnetic moments. The model developed here was also able to capture well the temperature dependence of the experimentally measured molecular spin valve magneto-conductance as well as its dependence on bias voltage in the static field regime